Electroluminescent display and electronic device comprising such a display

ABSTRACT

The invention relates to an electroluminescent display comprising a first display pixel and a second display pixel formed on a substrate. The display pixels comprise a first electrode deposited on or across the substrate, an electroluminescent layer and a second reflective electrode. The first and second display pixels are separated by a region comprising at least one insulating structure. The insulating structure is adapted to suppress the output of light at the second display pixel reflected at the second reflective electrode, which light is incident from at least the first display pixel and/or the substrate. The insulating structure reduces crosstalk of light between the first and second or further display pixel and can be easily integrated in the manufacturing process of the electroluminescent display.

The invention relates to an electroluminescent display comprising atleast a first display pixel and a second display pixel formed on asubstrate, said first and second display pixels comprising at least:

-   a first electrode deposited on or across said substrate,-   an electroluminescent layer, and-   a second reflective electrode,    wherein said first display pixel and said second display pixel are    separated by a region comprising at least one insulating structure.    Moreover, the invention relates to an electronic device comprising    such an electroluminescent display.

U.S. Pat. No. 5,989,785 discloses an electroluminescent devicecomprising display pixels formed on a substrate comprising luminescentregions sandwiched between two electrodes. The light output of aluminescent region can be influenced by the light output of anotherluminescent region, i.e. crosstalk of light. The crosstalk of lightbetween the luminescent regions is minimised by isolating theluminescent regions by means of a dielectric film. The refractive indexof the film is chosen to be totally reflecting the light incident from aluminescent region back into the same luminescent region.

However, in many instances crosstalk of light between the display pixelsin prior-art electroluminescent displays is still manifest. Crosstalk oflight can eventually result in the presence of ghost images on theelectroluminescent display, i.e. individual display pixels seem to be‘on’ while they are not activated by the display control means.Moreover, attempts to minimise crosstalk by adapting the structure ofthe display pixels has resulted in many additional manufacturing steps.

It is an object of the invention to provide an electroluminescentdisplay that substantially reduces crosstalk of light between thedisplay pixels due to light emanating from adjacent pixels and/orambient light from outside the display.

This object is achieved by providing an electroluminescent display,which is characterized in that said insulating structure is adapted tosuppress the output of light at said second display pixel reflected atsaid second reflective electrode, which light is incident from at leastsaid first display pixel and/or said substrate.

This insulating structure suppresses, reduces or even eliminates thecrosstalk of light between display pixels as a result of reflection atthe second reflective electrode and thereby reduces the possibility ofghost images on the electroluminescent display.

In a preferred embodiment of the invention, the insulating structurecomprises at least one edge near or along said second display pixel.Such an edge can e.g. be created by accommodation of the display pixelsin holes formed in an insulating layer. This embodiment has theadvantage that creation of such an insulating structure does not lead toan additional step in the manufacturing process of theelectroluminescent display. The insulating structure may exhibitslanting side walls towards at least one of the display pixels having anangle Φ towards a display pixel. In choosing the angle of the slantedside wall with the substrate carefully, the crosstalk of light betweenthe display pixels via the second electrode can be effectivelysuppressed, depending on the desired viewing angle. In a preferredembodiment, the angle Φ is larger than 40°, because in that case thecrosstalk of light is effectively suppressed for every viewing angle.

In a preferred embodiment of the invention, the insulating structure ismade at least partly of a material having a high refractive index. Theinsulating structure is preferably made of TiO₂ or SnO₂. Replacing aconventional dielectric layer by such a dielectric insulating layer witha higher refractive index does not lead to an additional manufacturingstep for such an electroluminescent device, while crosstalk of lightbetween the display pixels is suppressed.

In a preferred embodiment of the invention, the slanting side wall ofthe insulating structure comprises a roughened surface or a curvedsurface. Such a structure can be easily obtained and provides aneffective way of reducing crosstalk of light between the display pixelsof the electroluminescent display.

Except for adapting the angle, material or surface of the side wall ofthe insulating structure, light-absorbing means can also be used toprevent crosstalk of light between the display pixels. In a preferredembodiment of the invention, the insulating structure compriseslight-absorbing particles. Moreover, an absorbing grid, e.g. a blackmatrix, can be deposited underneath the slanting side wall of theinsulating layer. Also the second electrode can be partially removed andreplaced by a light-absorbing material. The embodiments comprisinglight-absorbing materials are simple with regard to manufacturing andprovide effective suppression of the crosstalk of light between thedisplay pixels of the electroluminescent display.

U.S. Pat. No. 6,901,195 discloses an electroluminescent displaycomprising reflectors for reducing crosstalk of light between thevarious devices of the electroluminescent display. Manufacturing of suchan electroluminescent display is complicated and requires additionalprocess steps and components as compared to the electroluminescentdisplay according to the invention.

It will be appreciated that the previous embodiments or aspects of theprevious embodiments of the invention can be combined.

The embodiments of the invention will be described in more detail belowwith reference to the attached drawing, in which

FIG. 1 is a cross-section of a conventional active matrixelectroluminescent display.

FIGS. 2A-2G show various embodiments of the invention.

FIG. 3 shows an example of the embodiment of the invention illustratedin FIG. 2A.

FIGS. 4A and 4B show the results of calculations performed for theembodiment of the invention illustrated in FIG. 2A.

FIG. 1 is a part of a cross-section of a conventional active matrixluminescent display (not to scale). The active matrix display comprisesa substrate 1 carrying first electrodes 2, an insulation layer 3, anorganic luminescent layer 4 and a second electrode 5. In thisconfiguration, the electroluminescent display exhibits various displaypixels 6, 7 arranged in rows and columns. The electroluminescent displayand/or the display pixels may comprise several additional layers,metallic layers (e.g. for providing capacitors), further insulatinglayers (e.g. for defining cross-overs) and semiconducting layers (e.g.for providing thin-film transistors).

The substrate 1 is preferably made of a transparent material such asglass or plastic. The thickness of the substate is e.g. 700 μm. Thetransparent substrate 1 is covered by the first electrodes 2, at leastat the sites where the display pixels 6, 7 are to be accommodated. Thefirst electrodes 2 are formed on the substrate by a deposition process,such as sputtering. These first electrodes 2 are preferably transparentwith respect to the light to be generated in the luminescent layer 4.Typically, these first electrodes 2 are made from Indium-Tin-Oxide(ITO), but different conductive and transparent materials, such asconductive polymers (polyaniline (PANI) or apoly-3,4-ethylenedioxythiophene (PEDOT)) can also be applied. During themanufacturing of the electroluminescent display, a (dielectric)insulating layer 3 is deposited on top of the first electrodes 2 andsubsequently removed on the sites where the display pixels 6 and 7 areto be formed. In this example, the dielectric insulating layer 3 wasmade of SiN and has a thickness of 0.5 μm. In fact, the insulating layer3 separates the display pixels 6 and 7 by the formation of holes in theinsulating layer exhibiting slanting side walls 8, 9 towards thesedisplay pixels. The width of the display pixels 6, 7 is e.g. 50 μm andthe display pixels are separated by a region over a distance of 30 μm ofwhich the slanting side walls 8, 9 take 5 μm each. It is noted that theinsulating layer 3 may extend across the edges of the first electrodes 2next to the slanting side wall 8, provided that electrical contact withthe first electrode 2 can be established. In this case, the width of theinsulating layer or structure 3 is thus larger than the width of theregion separation of the display pixels 6 and 7. The first electrodes 2or insulating layer 3 are covered by the electroluminescent layer 4 or alayer comprising an electroluminescent material, such as certain organicmaterials like poly-p-phenylenes (PPV) or derivatives thereof. Theelectroluminescent layer 4 can be deposited by using vacuum deposition,chemical vapour deposition or fluid-using techniques such asspin-coating, dip-coating or inkjet printing. The electroluminescentlayer 4 is covered by the second electrode 5, at least at the siteswhere the display pixels 6, 7 are to be formed. The second electrode isa metal and is highly reflective.

It is noted that while FIG. 1 is a cross-section of an active matrixmonochrome electroluminescent display, the invention and its advantagesapply equally well to passive matrix electroluminescent displays,segmented displays and colour displays. In passive matrix displays, thedisplay pixels are usually separated by photoresist layers orstructures. In the text below, embodiments of the invention will bedescribed in detail with respect to a monochrome active matrix displayas illustrated in FIG. 1.

In operating the electroluminescent display shown in FIG. 1, voltagescan be applied to the various display pixels 6, 7 by display controlmeans (not shown). If no voltage is applied to the electrodes 2, 5, nolight is generated in the luminescent layer 4 and the pixel is ‘off’ asholds for pixel 7 in FIG. 1. If a voltage is applied to the luminescentlayer 4, as holds for pixel 6, light is generated in this layer 4, i.e.the pixel is ‘on’. This light leaves the display pixel 6 through thetransparent first electrode 2 and the transparent substrate 1 into theair, resulting in a direct image of the display pixel 6, indicated bythe ray 10.

The light generated at the display pixel 6 is emitted Lambertianally,i.e. the light emission is distributed equally in each direction.Therefore, some light also traverses the substrate 1 as indicated by therays 11. These rays 11 will be reflected internally (TIR) at thesubstrate-air interface and subsequently pass (i.e. crosstalk) to anadjacent display pixel 7. As illustrated in FIG. 1, the rays 11′ arereflected at the second reflective electrode 5 that acts as a mirror tothese rays 11′. The reflected rays 11 then leave the display pixel 7 asrays 11″ because of the inclination of the second reflective electrode5, resulting in an image of the display pixel 7. The inclination of thesecond electrode 5 is due to the slanting side walls 8, 9 of the holesin the insulating layer 3 for accommodating the display pixels 6, 7.Thus, while display pixel 7 is ‘off’, an image of this pixel is presentdue to crosstalk of light initiated at a pixel that is ‘on’ andreflected within the electroluminescent display. This image willhereinafter be referred to as a ghost image. Such a ghost image may alsoresult from light that originates from outside the electroluminescentdisplay, i.e. ambient light, and is reflected by the second electrode 5.Crosstalk of light between the display pixels 6, 7 creates a reducedcontrast that is dependent on the viewing angle and may result indiscolouration in colour displays due to mixing of light from thevarious colour (RGB) display pixels.

FIG. 2 shows various embodiments of the invention wherein theelectroluminescent display comprises an insulating structure that isadapted to suppress the crosstalk of light between display pixels 6, 7due to reflection of light at the second electrode 5. It will beappreciated that the display pixels are not necessarily adjacent to eachother as is shown in FIG. 1. The light 11′ may originate as well orsolely from a display pixel or display pixels that are further away,i.e. not adjacent to the second display pixel 7.

FIG. 2A shows a preferred embodiment wherein the slanting side wall 8 ofthe insulating layer 3 is properly shaped with respect to the angle Φmade by the slanting side wall 8 with respect to the surface of thesubstrate 1. It has been found that in practical situations as describedbelow, an angle Φ of more than 40° substantially eliminates or reducesundesired reflection from the second electrode 5 of the light rays 11′,resulting in a ghost image from the display pixel 7 for all viewingangles. This embodiment will be discussed in more detail below.

FIG. 2B shows a preferred embodiment of the invention wherein theinsulating layer 3 has a sufficiently high refractive index. Forexample, TiO₂ (n=2,5) or SnO₂ (n=2) may be used for this dielectriclayer. The high refractive index results in an increased refraction atthe interface of the substrate 1 and the insulating layer 3, therebyeffectively suppressing crosstalk of light between the display pixels 6,7.

FIG. 2C shows a preferred embodiment of the invention wherein thesurface 12 of the slanting side wall 8 of the insulating layer 3 hasbeen roughened. Such a roughening can be easily obtained by reactive ionetching (RIB). Alternatively, a rough surface 12 can be obtained bydepositing various thin insulating layers with decreasing width parallelto the substrate 1 so as to obtain a step-like insulating layer 3. Theadvantage over RIE of such an approach is the avoidance of pin-holes inthe insulating layer 3. The effect of the rough surface 12 of theslanting side wall 8 is that TIR-light 11′ from the substrate-airinterface is diffused instead of reflected by the second electrode 5,resulting in a substantial decrease of the amount of light 11″ for theghost image of the display pixel 7.

FIG. 2D shows a preferred embodiment of the invention wherein thesurface 13 of the side wall of the insulating layer 3 is properlycurved, convex, so as to prevent crosstalk of light to the display pixel7. The curvature of the side wall 13 can be obtained by isotropicetching of the insulating layer 3.

In FIGS. 2A to 2D, the insulating structure is implemented by makingadjustments for the shape or material of (parts of) the insulating layer3. These adjustments can be very easily implemented in the manufacturingprocess of the electroluminescent displays, because no or only fewadditional process steps are required. These insulating structuresprovide an effective way of suppressing the appearance of ghost imagesof display pixels 7 due to light from other display pixels 6 or ambientlight. The contrast of the display pixels 6, 7 is optimal anddiscolouration in colour displays is eliminated.

A second approach to an effective elimination of crosstalk between thevarious display pixels 6, 7 or ambient light effects relates to theapplication of light-absorbing materials. Various embodiments of thisapproach are shown in FIGS. 2E-2G.

FIG. 2E shows a preferred embodiment of the invention wherein theinsulating layer 3 comprises light-absorbing particles such as carbonparticles. The light-absorbing particles provide effective crosstalkprevention means in that the TIR-rays 11′ are absorbed by the particlesprior to or after reflection at the second electrode 5, as a result ofwhich substantially no light 11″ leaves the insulating layer 3.

FIG. 2F shows a preferred embodiment of the invention wherein anabsorbing grid 14, i.e. a black matrix, has been applied underneath theslanting side wall 8 of the insulating layer 3. TIR-rays 11′ areprevented by the black matrix 14 from entering or leaving the insulatinglayer 3 as rays 11″ so that crosstalk between the display pixels 6, 7 issuppressed or optimally eliminated.

Finally, FIG. 2G shows a preferred embodiment of the invention whereinthe second reflective electrode 5 has been partially removed above theslanting side wall of the insulating layer 3. It will be appreciatedthat application of voltages to the display pixels 6, 7 should still bepossible. Preferably, the bare parts of the insulating layer are coveredby an absorbing material 15. In this embodiment, the effect of thesecond electrode 5 acting as a mirror is significantly reduced, as aresult of which crosstalk between the display pixels 6, 7 is reduced.

FIG. 3 shows three cases A-C, referring to FIG. 2 A, wherein the angle Φof the slanting side wall 8 of the insulation layer 3 is varied. θ₂ isthe angle of refraction of the TIR-rays 11′ at the interface of thesubstrate 1 and the insulating layer 3; the angle θ₅ refers to theviewing angle with respect to the normal of the substrate 1. In A, thecase 0<Φ<θ₂/2 is shown and θ₅>0; in B, θ₂/2<Φ<θ₂ and θ₅<0 and in C, Φ>θ₂while no light output is present.

Since θ₁ ^(lim)<θ₁<90° and θ₄ ^(lim)<θ₄<90° must hold for total internalreflection at the substrate-air interface, application of Snell's lawresults in the expression Φ>Φ^(lim)=(θ₂ ^(max)+θ₂ ^(min))/2, for theminimum angle n of the slanting side wall 8 of the insulating layer 3 soas to prevent crosstalk of light between the various display pixels 6,7. θ₂ ^(max) and θ₂ ^(min) are the maximum and minimum angles ofrefraction at the interface of the substrate 1 and the insulating layer3 relating to the maximum and minimum angle θ₁ of incidence,respectively, of the light 11. Taking n=1 as the refractive index n forair, n=1.5 for the substrate 1 composed of glass and SiO₂ and n=2 forthe insulating layer 3, this results in a minimum angle Φ^(lim) ofapproximately 39° for the slanting side wall 8.

Further analysis of the embodiment of FIG. 2A results in the graphsshown in FIGS. 4A and 4B. FIG. 4A shows a range R of angles Φ for whicha ghost image comes from the display at a particular viewing angle ↓⁵.An angle Φ of more than 40° for the slanting wall of the insulatinglayer 3 is sufficient to avoid unwanted reflections at the secondelectrode so that no ghost image is generated at any of the viewingangles θ₅. FIG. 4B provides an alternative representation of thisresult, wherein the graphs (A), (B) and (C) correspond to the cases A-Cshown in FIG. 3.

For the purpose of teaching the invention, preferred embodiments of thedisplay device and the electronic device comprising such a displaydevice have been described above. It will be apparent to the personskilled in the art that other alternative and equivalent embodiments ofthe invention can be conceived and reduced to practice without departingfrom the true spirit of the invention, the scope of the invention beingonly limited by the claims.

1. An electroluminescent display comprising at least a first displaypixel (6) and a second display pixel (7) formed on a substrate (1), saidfirst and second display pixels comprising at least: a first electrode(2) deposited on or across said substrate (1), an electroluminescentlayer (4), and a second reflective electrode (5), wherein said firstdisplay pixel (6) and said second display pixel (7) are separated by aregion comprising at least one insulating structure (3), characterizedin that said insulating structure (3) is adapted to suppress the outputof light (11″) at said second display pixel (7) reflected at said secondreflective electrode (5), which light (11″) originates from light (11′)incident from at least said first display pixel (6) and/or saidsubstrate (1).
 2. An electroluminescent display as claimed in claim 1,wherein said insulating structure (3) comprises at least one edge nearor along said second display pixel (7).
 3. An electroluminescent displayas claimed in claim 2, wherein said edge comprises at least one slantingside wall (8) having an angle 4) towards said second display pixel (7).4. An electroluminescent display as claimed in claim 3, wherein saidangle Φ is larger than (θ₂ ^(max)+θ₂ ^(min))/2, with θ₂ ^(max) and θ₂^(min) being the maximum and minimum angles of refraction at theinterface of the substrate (1) and the insulating structure (3),respectively.
 5. An electroluminescent display as claimed in claim 3 or4, wherein said angle Φ is chosen to be dependent on a desired viewingangle θ₅ in accordance with FIG. 4A.
 6. An electroluminescent display asclaimed in claim 3, 4 or 5, wherein said angle Φ is larger than 40°. 7.An electroluminescent display as claimed in claim 1, wherein saidinsulating structure (3) is made of a material with a refractive indexwhich is equal to or higher than 2.0.
 8. An electroluminescent displayas claimed in claim 7, wherein said insulating structure (3) comprisesTiO₂ or SnO₂.
 9. An electroluminescent display as claimed in claim 3,wherein said insulating structure (3) comprises a roughened surface (12)of said slanting side wall (8).
 10. An electroluminescent display asclaimed in claim 3, wherein said insulating structure (3) comprises acurved side wall (13).
 11. An electroluminescent display as claimed inclaim 1 or 2, wherein said insulating structure (3) compriseslight-absorbing particles.
 12. An electroluminescent display as claimedin claim 3, wherein said insulating structure (3) comprises alight-absorbing grid (14) suitably deposited underneath said slantingside wall (8).
 13. An electroluminescent display as claimed in claim 1or 2, wherein said insulating structure (3) comprises a light-absorbingmaterial (15) which partly replaces said second reflective electrode(5).
 14. An electroluminescent display as claimed in claim 1, whereinsaid insulating structure (3) is adapted in accordance with acombination of any one of the preceding claims.
 15. An electronic devicecomprising an electroluminescent display as claimed in any one of thepreceding claims.